The pressure rating would remain the same as -20 deg F because the mechanicals will not change significantly below -20 deg F. What is required is to prove suitability for service at -40 deg F by impact testing.
Read below from 16.34
2.3.2 Low Temperature. The pressure rating for service
at any temperature below −29°C (−20°F) shall be
no greater than the rating shown in Table 2 for −29°C
(−20°F). Some of the materials listed in Table 1, notably
some carbon steels, may undergo a decrease in ductility
when used at low temperatures to such an extent as to
be unable to safely resist shock loading, sudden change
of stress, or high stress concentration. Some codes or
regulations may require impact testing for applications
even where temperatures are higher than −29°C (−20°F).
When such requirements apply, it is the responsibility
of the user to ensure these requirements are communicated
to the manufacturer prior to the time of purchase.
